Conventionally, in order to respond to a demand for miniaturization and high-performance of high-frequency components (RF components) for use in mobile phones, developments of MEMS switches as high-frequency (RF) switches have been in progress by use of MEMS (Micro Electro Mechanical Systems) techniques. The MEMS switches have, as features thereof, lower loss, higher isolation, good distortion properties, and so on as compared with conventional semiconductor switches.
Various types of MEMS switches having different structures have conventionally been proposed (see Japanese National Publication of International Patent Application No. 2005-528751 and Japanese Laid-open Patent Publication Nos. 2005-293918 and 2006-210530).
FIG. 11 is a plan view illustrating a conventional MEMS switch 80j, and FIGS. 12A-12C are cross sections of the MEMS switch 80j. To be specific, FIGS. 12A-12C are cross sectional views of the MEMS switch 80j taken along the line J1-J1, the line J2-J2, and the line J3-J3 in FIG. 11, respectively.
Referring to FIGS. 11-12C, the MEMS switch 80j is formed of a substrate 81 on which a lower contact electrode 82, an upper contact electrode 83, a lower driving electrode 84, an upper driving electrode 85, a ground electrode 86, and so on are formed. The lower contact electrode 82 and the lower driving electrode 84 are integrated with a movable portion KB that constitutes a cantilever.
The substrate 81 is a Silicon-on-Insulator (SOI) substrate. A slit ST is formed on an active layer of the SOI substrate; thereby to define the movable portion KB. The lower contact electrode 82 and the lower driving electrode 84 are formed on the active layer by plating.
The lower contact electrode 82 and the upper contact electrode 83 are used as a high-frequency signal line. The high-frequency signal line forms a coplanar line structure along with the upper driving electrode 85 and the ground electrode 86 that are provided to interpose the high-frequency signal line therebetween, which results in the low transmission loss.
The upper driving electrode 85 is connected to the ground. When a driving voltage VD is applied between the upper driving electrode 85 and the lower driving electrode 84, an electrostatic attractive force is generated therebetween with which the lower driving electrode 84 is attracted toward and moved to the upper driving electrode 85. In this way, the movable portion KB that is integrated with the lower driving electrode 84, and the lower contact electrode 82 move, and the lower contact electrode 82 touches the upper contact electrode 83 so that the contacts close. At this time, if the driving voltage VD is set at zero, the contacts of the lower contact electrode 82 and the upper contact electrode 83 separate from each other due to the elasticity of the movable portion KB.
In the MEMS switch 80j having a conventional structure discussed above, when a driving voltage is applied to the lower driving electrode 84, a leakage current Ia flows from the lower driving electrode 84 through the active layer of the movable portion KB to the lower contact electrode 82 functioning as the high-frequency signal line.
Even in the case of the movable portion KB made of high-resistance silicon, the leakage current Ia is, for example, approximately 10 μA when the driving voltage VD is 40 V. In such a case, power consumption due to the leakage current Ia is 400 μw. The level of the power consumption is not a negligible level in, for example, a portable terminal.
The leakage current Ia is eventually carried to the contacts of the high-frequency signal line, which is probably a cause of contact sticking.